Lumiphors (also known as lumiphoric materials) are commonly used with electrically activated emitters to produce a variety of emissions such as colored (e.g., non-white) or white light (e.g., perceived as being white or near-white). Electrically activated emitters may be utilized to provide white light (e.g., perceived as being white or near-white), and have been investigated as potential replacements for white incandescent lamps. Such emitters may have associated filters that alter the color of the light and/or include lumiphors that absorb a portion of emissions having a first peak wavelength emitted by the emitter and re-emit the light having a second peak wavelength that differs from the first peak wavelength. Phosphors, scintillators, and lumiphoric inks are common lumiphors. Light perceived as white or near-white may be generated by a combination of red, green, and blue (RGB) emitters, or, alternatively, by combined emissions of a blue light emitting diode (LED) and a lumiphor such as a yellow phosphor. In the latter case, a portion of emissions from the blue LED passes through the phosphor, while another portion of the blue LED emissions is absorbed by the phosphor and downconverted to yellow emissions, and the resulting combination of blue and yellow light is perceived as white. Another approach for producing white light is to stimulate phosphors or dyes of multiple colors with a violet or ultraviolet LED source.
LEDs (including both organic and inorganic light emitting diodes) are electrically activated emitters that convert electric energy to light, and generally include one or more active layers of semiconductor material adjacent to doped layers. When bias is applied across doped layers, holes and electrons are injected into one or more active layers, where they recombine to generate light that is emitted from the device. Laser diodes are solid state emitters that operate according to similar principles.
A representative example of a white LED lamp includes a package of a blue LED chip (e.g., made of InGaN and/or GaN) combined with a lumiphor such as a phosphor (typically YAG:Ce) that absorbs at least a portion of the blue light (first peak wavelength) and re-emits yellow light (second peak wavelength), with the combined yellow and blue emissions providing light that is perceived as white or near-white in character. If the combined yellow and blue light is perceived as yellow or green, it can be referred to as ‘blue shifted yellow’ (“BSY”) light or ‘blue shifted green’ (“BSG”) light. Addition of red spectral output from an emitter or lumiphor may be used to increase the warmth of the aggregated light output. The addition of one or more red LEDs (e.g., (Al,In,Ga)P-based) to a blue (e.g., GaN-based) LED-based (e.g., BSY) lighting device improves color rendering and better approximates light produced by incandescent lamps. Emitters or lumiphors of other colors may be used, such as disclosed in U.S. Patent Application Publication No. 2007/0223219 to Medendorp, Jr., et al. As indicated previously, it is known to supplement emissions from primary blue LEDs and yellow phosphors with red spectral output, such as may be generated by supplemental red LEDs or red phosphors. Use of red LEDs is often preferable to use of red phosphors in such context, such as to promote greater efficacy and/or controllability. The use of red supplemental LEDs in combination with high-power primary blue LEDs, however, creates additional problems. Red LEDs include active regions typically formed of (Periodic Table) Group III phosphide (e.g., (Al,In,Ga)P) material, in contrast to blue LEDs, which include active regions typically are formed of Group III nitride materials (e.g., represented as (Al,In,Ga)N, including but not limited to GaN). Although luminous efficiency varies with respect to temperature for LEDs of all materials, the efficiency if Group III phosphide based solid state emitters declines at a greater rate than Group III nitride based solid emitters at elevated temperatures. In devices including both red and blue LEDs, heat emanating from the blue LEDs will increase the temperature of the red LEDs. To maintain a relatively constant color point utilizing a device including a Group III-nitride-based blue LED (e.g., as part of a BSY emitter) and Group III-phosphide based red LED, ratio of currents supplied to the red LEDs relative to the blue LEDs must be altered (i.e., increased) as temperature increases because of the different temperature responses of the blue and red LEDs. Ultimately, reduction of luminous efficiency of red LEDs results in reduction in total flux from the combination of emitters at a desired color point, or results in a lack of control of color point at higher flux values, thereby limiting utility of the device.
Many modern lighting applications require high power emitters to provide a desired level of brightness. High power emitters can draw large currents, thereby generating significant amounts of heat. Limitations associated with binding a lumiphor (e.g., phosphor) to an emitter surface generally restrict the total amount of radiance that can be applied. Lumiphor binding materials and/or lumiphors tend to change color (e.g., darken) after extended periods of exposure to elevated temperatures. In order to increase reliability and prolong useful service life of a lighting device including a lumiphor, the lumiphor may be physically separated from an electrically activated emitter. Separation of the lumiphor element permits the electrically activated emitter to be driven with higher current and thereby produce a higher radiance without excessively heating the lumiphor element. Although various lighting devices utilizing remote lumiphors are known (e.g., such as disclosed in U.S. Patent Application Publication No. 2005/0270775), such devices typically have limited color rendering ability and/or limited tenability to achieve a desired color point.
Another difficulty associated with use of multiple emitters of different colors is achieving desirable uniformity of color of the aggregated emissions therefrom, without unduly reducing total flux of the resulting emissions.
In consequence, the art continues to seek improvements in light emitting devices that include many of the advantages associated with use of high output emitters, but which also have the capacity to produce warmer light and improved color rendering at high flux. It would also be desirable to provide tunabilty of the color and chromaticity of light emitted by a light emitting device.